The present invention relates to magnetic bearing devices, and more particularly to a magnetic bearing device for magnetically levitating a rotary body by contactlessly supporting the body with a plurality of magnetic bearings with respect to an axial direction and two radial directions orthogonal to each other and to the axial direction.
Already known as such magnetic bearing devices are those having five control axes, i.e., a control axis in an axial direction and two control axes respectively in two radial directions at each of two locations in the axial direction.
Magnetic bearing devices of the five-axis control type comprise an axial magnetic bearing for contactlessly supporting a rotary body with respect to the direction of axial control axis, two radial magnetic bearings for contactlessly supporting the rotary body with respect to the two directions of radial control axes orthogonal to each other at each of the two locations in the axial direction, and protective bearings (touchdown bearings) serving as mechanical restraining means for mechanically determining the movable ranges of the rotary body in the axial direction and the radial directions. The axial magnetic bearing comprises a pair of electromagnets so arranged as to hold the rotary body at opposite sides thereof in the direction of axial control axis for magnetically attracting the body. Each of the radial magnetic bearings comprises a pair of electromagnets so arranged as to hold the rotary body at opposite sides thereof in the direction of radial control axis concerned for magnetically attracting the body. Furthermore, the magnetic bearing device comprises a position detector serving as position detecting means for detecting the position of the rotary body with respect to the direction of each of the five control axes, and an electromagnet controller serving as electromagnet control means for controlling the pair of electromagnets for the control axis based on the result of detection of the position by the position detector. The position detector in the direction of axial control axis comprises one axial position sensor opposed to the end face of the rotary body to be detected from the direction of axial control axis. The position detector in the direction of each radial control axis comprises a pair of radial position sensors opposed to the rotary body and so arranged as to hold the body at opposite sides thereof in the direction of the control axis. The electromagnet controller comprises a proportional operation unit, differential operating unit and integral operating unit.
With respect to the direction of each control axis, the magnetic bearing device described has a mechanical central position for the movable range determined by the protective bearings, a magnetic central position relative to the positions of the electromagnets of the magnetic bearing, and central position relative to the sensors, i.e., relative to the positions of the position sensors. The mechanical central position in the direction of each control axis is the position of center of the movable range determined by the protective bearings. The magnetic central position with respect to the direction of each control axis is the position of the center of the pair of electrode magnets as arranged in the direction of the control axis. The central position relative to the sensor in the direction of the axial control axis is such that the gap (clearance) between the position detecting end face of the rotary body and the axial position sensor has a predetermined value. The central position relative to the sensors in the direction of each radial control axis is the position of center of the pair of radial position sensors as arranged in the direction of the control axis. The magnetic bearing device is so designed that the mechanical central position, the magnetic central position and the central position relative to the sensors are all in coincidence, whereas an error is likely to occur between the centers owing to manufacturing errors or assembling errors.
With the conventional magnetic bearing device, the electromagnets of the magnetic bearings are so controlled that the rotary body will be held at the central position relative to the sensors which is the designed central position, that is, the center of the rotary body will be at the central position relative to the sensors. Accordingly, the rotary body can not be held at the mechanical central position if the central position relative to the sensors is not in coincidence with the mechanical central position. When the difference of the mechanical central position of the rotary body from the central position relative to the sensors is great in this case, the clearance between the rotary body and the protective bearing diminishes locally to result in troubles.
An object of the present invention is to provide a magnetic bearing device capable of magnetically levitating a rotary body approximately at the mechanical central position of the device easily.
The present invention provides a magnetic bearing device for magnetically levitating a rotary body by contactlessly supporting the body with magnetic attraction of pairs of electromagnets with respect to an axial direction and two radial directions orthogonal to each other and to the axial direction, the rotary body having movable ranges in the three supporting directions determined by mechanical restraining means, the magnetic bearing device being characterized in that the device comprises a pair of electromagnets so arranged as to hold the rotary body at opposite sides thereof in the direction of each of control axes in the respective three supporting directions, means for detecting the position of the rotary body in the direction of the control axis and electromagnet control means having at least an integral operation unit for controlling the electromagnets based on the result of detection of the position by the position detecting means, the electromagnet control means comprising a target levitated position setting means for setting as a target levitated position of the rotary body in the direction of the control axis the position of the rotary body corresponding to the median of an integral output which is the output of the integral operation unit when the rotary body is magnetically levitated in the vicinity of one of limit positions in the direction of the control axis determined by the mechanical restraining means and an integral output of the integral operation unit when the rotary body is magnetically levitated in the vicinity of the other limit position.
The pair of electromagnets to be used for each control axis are usually identical in characteristics. The electromagnet control means supplies to each electromagnet an energizing current comprising the combination of a predetermined steady-state current and a control current which varies depending on the position of the rotary body in the direction of the control axis. The steady-state current values for the pair of electromagnets are equal to each other, and the control currents therefor are equal to each other in absolute value but opposite in sign.
In the case where no force other than the magnetic attraction of the electromagnets acts on the rotary body as in the case of a horizontal control axis, and when the rotary body is contactlessly supported at a position with respect to the direction of the control axis, the magnetic attracting forces of the two electromagnets on the rotary body are equal to each other. Further the magnetic attraction of each electromagnet is in proportion to the square of the magnitude of the energizing current of the electromagnet and in inverse proportion to the size of the gap between the electromagnet and the rotary body. When the rotary body is supported at the magnetic central position, the gaps between the rotary body and the respective electromagnets are equal to each other in size, with the result that the energizing current values of the respective electromagnets are equal to each other. Thus, the control current values for the respective electromagnets are zero. When the rotary body is supported as shifted toward either direction from the magnetic central position, the gaps between the rotary body and the electromagnets are different from each other in size, so that the energizing current values of the electromagnets differ from each other. Accordingly the control current values for the electromagnets differ from each other. The control current value of each electromagnet is proportional to the displacement of the rotary body from the magnetic central position. Further the control current value of one of the electromagnets is in proportion to the output of the integral operation unit of the electromagnet control means, i.e., the integral output. When the position where the rotary body is supported is shifted from one limit position to the other limit position, therefore, the integral output varies linearly. For this reason, the position of the rotary body corresponding to the median of the integral output obtained when the rotary body is magnetically levitated in the vicinity of one limit position and the integral output obtained when the rotary body is magnetically levitated in the vicinity of the other limit position is approximately the mechanical central position, with the result that by taking the position corresponding to the median as a target levitated position, the rotary body can be magnetically levitated approximately at the mechanical central position.
In the case where the rotary body is subjected to gravity in addition to the magnetic attraction of electromagnets as in the case of a vertical control axis, and when the rotary body is contactlessly supported at a position in the direction of the control axis, the upward magnetic attraction afforded by the upper electromagnet is in balance with the combination of the downward magnetic attraction of the lower electromagnet and gravity. With the rotary body supported at the magnetic central position, therefore, the control current for the upper electromagnet has a positive value, and the control current for the lower electromagnet is of a negative value, so that the integral output is not zero. Even in this case, however, the integral output varies linearly if the position where the rotary body is supported is shifted from one limit position to the other limit position. As in the foregoing case, the rotary body can accordingly be magnetically levitated approximately at the mechanical central position by taking as the target levitated position a position corresponding to the median of the integral output obtained when the rotary body is magnetically levitated in the vicinity of one limit position and the integral output obtained when the rotary body is magnetically levitated in the vicinity of the other limit position.
The same is true of the case wherein the control axis is oblique; regardless of the posture of the rotary body as installed, the body can be magnetically levitated approximately at the mechanical central position.
The magnetic bearing device is usually divided into a machine main body comprising a rotary body, electromagnets of magnetic bearings, protective bearings and position sensors included in position detecting means, and a controller comprising electromagnet control means, a sensor circuit included in the position detecting means for driving the position sensors and calculating the position of the rotary body based on the outputs of the position sensors, etc. The main body and the controller are interconnected by a cable. Further while the magnetic bearing device is in use, there arises a need to replace the controller by another one of the same type. Even if of the same type, machine main bodies are not always identical in the relationship between the mechanical central position, magnetic central position and central position relative to the sensors. However, the machine main bodies remain unchanged in the relationship between these three positions. Accordingly, insofar as the combination of the machine main body and the controller remains unchanged, a target levitated position may be set as described above when the magnetic bearing device is to be initiated into operation for the first time. It is then possible to subsequently magnetically levitate the rotary body approximately at the mechanical central position by using this target levitated position. If the combination of machine main body and controller is changed, however, the target levitated position is not set in the new controller, which therefore fails as it is to magnetically levitate the rotary body at the mechanical central position. Even in such a case, nevertheless, a target levitated position can be set in the manner described above to magnetically levitate the rotary body approximately at the mechanical central position.
When the magnetic bearing device of the present invention is to be initiated into operation for the first time, or when the combination of the machine main body and the controller is changed, the rotary body can be magnetically levitated approximately at the mechanical central position by the simple procedure of moving the rotary body from one of limit positions in the direction of control axis to the other limit position, regardless of the posture of the rotary body as installed. Accordingly it is unlikely that the clearance between the rotary body and mechanical restraining means will diminish locally as experienced in the prior art to result in various troubles.
For example, the target position setting means is adapted to position the rotary body at said one limit position, thereafter magnetically levitate the rotary body in the vicinity thereof, obtain the integral output at this time to store the output as a first limit position integral output in a memory, gradually shift the magnetically levitated position of the rotary body toward said other limit position, determine the position of the rotary body every time the rotary body is so shifted by a small distance at a time and the corresponding integral output for storage as an intermediate position and an intermediate position integral output in the memory, move the rotary body to said other limit position, thereafter magnetically levitate the rotary body in the vicinity thereof, obtain the integral output at this time for use as a second limit position integral output, determine the median of the first limit position integral output and the second limit position integral output, and select the output most proximate to the median from among the intermediate position integral outputs stored in the memory to determine the intermediate position corresponding to the selected intermediate position integral output as the target levitated position.